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  1. module Main where
  2.  
  3. --
  4. -- In order to use "do" notation, we need to declare instances
  5. -- of the Monad typeclass. In order to do that we must jump through
  6. -- some hoops:
  7. --    * We need a name for the datatype we are using (hence the new "Counter" type)
  8. --    * We also must declare instances of Functor and Applicative
  9. --      (this is something that changed in Haskell 7.10.
  10. --       See https://w...content-available-to-author-only...l.org/Functor-Applicative-Monad_Proposal )
  11. --      (Applicative's "pure" is the same as Monad's "return")
  12. -- 
  13.  
  14. -- Integer arithmetic with (+) and (/)
  15. data Term = 
  16.       N Int
  17.     | Plus Term Term
  18.     | Div Term Term
  19.  
  20. --
  21. -- Basic evaluator
  22. --
  23.  
  24. eval :: Term -> Int
  25. eval (N n) = n
  26. eval (Plus t1 t2) = eval t1 +     eval t2
  27. eval (Div t1 t2)  = eval t1 `div` eval t2
  28.  
  29. --
  30. -- Evaluator that counts the number of operations
  31. --
  32.  
  33. newtype Counter a = Counter (Int -> (Int, a))
  34.  
  35. unCounter :: Counter a -> (Int -> (Int, a))
  36. unCounter (Counter f) = f
  37.  
  38. instance Functor Counter where
  39.     -- fmap :: (a -> b) -> Counter a -> Counter b
  40.     fmap f = undefined
  41.  
  42. instance Applicative Counter where
  43.     -- pure :: a -> Counter a
  44.     pure n = undefined
  45.  
  46.     -- Counter (a -> b) -> Counter a -> Counter b
  47.     c1 <*> c2 = undefined
  48.  
  49. instance Monad Counter where
  50.     -- (>>=) :: Counter a -> (a -> Counter b) -> Counter b
  51.     m >>= f = undefined
  52.  
  53. runC :: Counter a -> Int -> (Int, a)
  54. runC s0 c = undefined
  55.  
  56. inc :: Counter ()
  57. inc = undefined
  58.  
  59. evalWithCount :: Term -> (Int, Int)
  60. evalWithCount t = runC (eval' t) 0
  61.   where
  62.     eval' :: Term -> Counter Int
  63.     eval' (N n) =
  64.         return n
  65.     eval' (Plus t1 t2) = do
  66.         n1 <- eval' t1
  67.         n2 <- eval' t2
  68.         inc
  69.         return (n1 + n2)
  70.     eval' (Div t1 t2) = do
  71.         n1 <- eval' t1
  72.         n2 <- eval' t2
  73.         inc
  74.         return (n1 `div` n2)
  75.  
  76. --
  77. -- Evaluator that catches division by zero
  78. --
  79. -- Returns (Ok n) in case of success
  80. -- Returns (Error "Division by zero") otherwise
  81. --
  82.  
  83.  
  84. data Result a = Error String | Ok a deriving Show
  85.  
  86. instance Functor Result where
  87.     -- fmap :: (a -> b) -> Result a -> Result b
  88.     fmap f r = undefined
  89.  
  90. instance Applicative Result where
  91.     -- pure :: a -> Result a
  92.     pure n = undefined
  93.  
  94.     r1 <*> r2 = undefined
  95.  
  96. instance Monad Result where
  97.     -- (>>=) :: Result a -> (a -> Result a) -> Result b
  98.     m >>= f = undefined
  99.  
  100. throw :: String -> Result a
  101. throw s = undefined
  102.  
  103. evalChecked :: Term -> Result Int
  104. evalChecked (N n) =
  105.     return n
  106. evalChecked (Plus t1 t2) = do
  107.     n1 <- evalChecked t1
  108.     n2 <- evalChecked t2
  109.     return (n1 + n2)
  110. evalChecked (Div t1 t2) = do
  111.     n1 <- evalChecked t1
  112.     n2 <- evalChecked t2
  113.     if n2 == 0 then
  114.         throw "Division by zero"
  115.     else
  116.         return (n1 `div` n2)
  117.  
  118. main = print (undefined::String)
  119.  
Runtime error #stdin #stdout #stderr 0s 0KB
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Standard input is empty
stdout
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Standard output is empty
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prog: Prelude.undefined
CallStack (from HasCallStack):
  error, called at libraries/base/GHC/Err.hs:79:14 in base:GHC.Err
  undefined, called at prog.hs:118:15 in main:Main
https://ideone.com/NDUwy6
language:
Haskell (ghc 8.4.4)
created:
5 years ago
visibility:
public

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